1. Field of the Invention
The present invention relates generally to low voltage lighting systems, and more specifically to systems which have long bus lines to deliver current to low voltage light bulbs located along the length of the bus lines.
2. Prior Art
At present, low-voltage lighting systems distribute current from a common power source to each light bulb in a parallel configuration. In other words, a wire (bus line) from the power source is connected to one end of several light bulbs along its length to distribute current in parallel to each light bulb. The product of the current and internal resistance of each light bulb is equal to the voltage drop across the light bulb; according to Ohm's law. Note that devices, concepts, and physical laws, such as Ohm's law, used in the theory of operation presented here are found in engineering texts such as William H. Hayt, Jr. and Jack E. Kemmerly, Engineering Circuit Analysis, Second Edition, McGraw-Hill Book Company, New York, 1971, and Jacob Millman, Ph.D. and Herbert Taub, Ph.D., Pulse, Digital, and Switching Waveforms, McGraw-Hill Book Company, New York, 1965. Another wire (return bus line) collects current from each light bulb and returns it back to the power source. If the resistance of the bus wire is zero, then no voltage drop occurs along the bus line and each light bulb has the same voltage applied across the bulb terminals, and thus the same electrical power or, alternatively, bulb brightness. However, in practice the bus wires have a finite (non-zero) internal resistance and thus experience a voltage drop along the length of the bus wires. This voltage drop means that the voltage applied across a light bulb will decrease with increasing distance from the power source. Thus light bulbs will be dimmer with increasing distance from the power source.
The voltage drop along the bus wires also limits the distance that light bulbs may be placed from the power source. However, the lighting effect is still one of light bulbs becoming dimmer with distance from the power source. The light bulbs are typically rated by maximum voltage applied across them and maximum electrical power. Dividing the square of the applied voltage by the power yields the internal electrical impedance of the light bulb. The impedance is a generalized electrical resistance that is well known to those skilled in the art, and includes the effects of electrical inductance and capacitance. In the case of a light bulb, a pure resistance may be assumed and will be used in the illustrative examples discussed here. In order to maintain voltage applied to each light bulb within its ratings, the power source is typically set to output a voltage near the limit (12 volts in a typical system). In this setting the light bulb closest to the power source may be driven with an applied voltage that is close to the source voltage in magnitude. However, the applied voltage may drop by several volts for a light bulb at the end of the bus line.
One possible method of compensation for an alternating current low-voltage system would be to introduce a transformer with an adjustable center tap in series with each light bulb and adjust the center tap to achieve the desired voltage across the light bulb. However, each time one transformer is adjusted it would influence the total current in the bus line and thus affect the voltage drop along the bus line. Therefore, this conventional approach would have to be iterative where each transformer is adjusted in turn and then the process repeated as needed until the voltages across each light bulb converges to the target voltage.
Another possible solution is a voltage regulator. However, voltage regulators in the form of integrated circuits apply to direct-current electrical systems, while low-voltage lighting systems typically are implemented using alternating current. Equivalents for more common alternating-current electrical circuits exist as servo-controlled transformers, but these are typically in large, relatively expensive chassis that are not practical for use at every bulb location in low-voltage lighting systems.
Sawase, et al. in “LED light source with an easily adjustable luminous energy,” (U.S. Pat. No. 5,150,016) teach a system that utilizes a finite number of resistors connected to each other electrically in parallel, and a plurality of fuses connected electrically in series with the respective resistors, with the parallel combination of these resistors with fuses connected electrically in series with each LED light source. The idea here is that any current larger than the current rating of a fuse burns out that fuse and electrically disconnects the associated resistor. Therefore, at each LED location, any remaining resistors act to limit the current to the LED from a voltage source. This approach is not very efficient in the sense that many components (fuses and resistors) are employed that end up essentially being discarded from use in each device as the result of burning out fuses. The technique of Sawase, et al. allows only for a relatively coarse adjustment of the current through each individual means for illumination (LED) due to the finite number of resistors that may be practically connected to each LED. Furthermore, in Sawase, et al., the current-limiting method employed is applied on an individual basis for each individual means for illumination. However, the issues that result in a need for adjustment of illumination of each individual means for illumination generally result from system-wide effects.